Matroska Video MIME Type

What does video/x-matroska mean?

The video/x-matroska MIME type identifies Matroska video files, normally with the .mkv extension. Matroska is a container, not a video codec. One MKV can hold video, multiple audio tracks, subtitles, chapters, tags, and attachments.¹²

That flexibility is useful for movies, multilingual releases, recorded presentations, and preservation masters. It also means the extension alone cannot tell you whether a particular device can play the file. A player must support the Matroska container and every codec needed by its tracks.

How to identify an MKV file

Matroska is built on EBML, the Extensible Binary Meta Language. A file starts with an EBML header whose element ID is 1A 45 DF A3, but that byte sequence is not unique to Matroska. Reliable identification also checks that the header's document type is matroska and validates the following structure.²⁴

The IANA registration therefore lists no standalone magic number for video/matroska.¹ Renaming another video to .mkv changes only its filename. It does not create a Matroska container.

Common Matroska extensions describe different kinds of content:

Use video/x-matroska or its replacement video/matroska for .mkv, not for every file based on Matroska.¹³

History and standardization

The Matroska project began on December 6, 2002 as a fork of the Multimedia Container Format project. It chose EBML so the container could evolve without making older readers reject every unfamiliar element.²⁵

Matroska version 4 is the current format version. Versions 1 through 3 are no longer maintained, but versioning rules are designed so newer files can remain usable by older readers when they avoid unsupported features.²

In October 2024, the IETF published RFC 9559 on the Standards Track. The RFC defines the container structure and established video/matroska as the registered media type, making video/x-matroska its deprecated alias.¹²

How to open a video/x-matroska file

For local playback, VLC lists Matroska among its supported input formats.⁶ MKVToolNix is useful when you need to inspect tracks, create an MKV, change container metadata, or extract a stream without treating the whole file as one video track.⁷

Native browser support is more dependent on both browser and codec. Firefox 145 added MKV support for commonly used combinations including AVC, HEVC, VP8, VP9, AV1, AAC, Opus, and Vorbis.⁸ Other browser and device combinations may accept only some Matroska files.

Matroska support in Poly

Poly recognizes .mkv files as video/x-matroska. For supported streams, you can:

• Browse with a representative thumbnail and timeline previews.
• Play the video in Poly and jump to a relevant moment.
• Find it by filename, spoken transcript, and sampled visual content.
• Review available details such as duration, dimensions, frame rate, codecs, color information, and audio tracks.
• Ask the Poly agent about supported visual content, speech, and metadata.

Speech search and transcript-based answers require a usable audio track with audible speech. Silent video can still benefit from visual search. Playback and content-aware features depend on the codecs carried by the MKV.

See Supported File Types for the compact video feature comparison.

MKV compared with WebM

WebM is based on Matroska, but it deliberately narrows the permitted codecs and features for web delivery.³ An .mkv file and a .webm file can share the same underlying EBML structure without being interchangeable.

Changing .mkv to .webm is safe only when the file already meets WebM's requirements and is remuxed correctly. Otherwise, compatible tracks must be selected or transcoded.

Converting MKV to MP4 or WebM

Choose a destination based on where the result must play:

Remuxing changes the container without changing compatible encoded streams, so it is fast and does not introduce generation loss. Transcoding decodes and re-encodes a track, which can improve compatibility but may reduce quality. Keep the original until you have checked every required audio, subtitle, chapter, and attachment.